This invention relates generally to drive trains and, more particularly, to drive train assemblies for outboard and stem drive marine engines.
Known outboard marine engines include a drive shaft which extends from an engine power head, through an exhaust case, and into an engine lower unit. The lower unit includes a gear case, and a propeller shaft extends through the gear case. The rotational axis of the propeller shaft is generally perpendicular to the rotational axis of the drive shaft. A bevel pinion drive gear affixed to the lower end of the drive shaft meshes with and drives two bevel driven gears diametrically opposed to each other and rotationally aligned with the propeller shaft. The driven gears have a same pitch diameter and a same number of teeth. Conventionally, the drive pinion and the driven gears have a common diametral pitch, or number of teeth per inch of pitch diameter.
A clutching member is slidingly connected to the propeller shaft and selectively engages one of the driven gears, thereby driving the propeller shaft in the same rotational direction as the engaged gear. One propeller shaft rotational direction associated with the first driven gear provides a forward thrust, and the other shaft rotational direction associated with the second driven gear provides reverse thrust.
The propeller is typically configured to maximize efficiency in forward gear since outboard marine engines are generally operated in forward gear relative to operation in a reverse gear. Consequently, at a given engine speed, the thrust generated by the propeller in forward gear generally is higher than the thrust generated by the propeller in reverse gear. To compensate for this lack of efficiency in reverse gear, a dual drive pinion arrangement could be utilized to provide for increasing the rotational speed of the propeller in reverse gear. In an outboard engine, however, such a dual pinion arrangement would result in an increased size of the engine lower unit, increased drag, additional parts, added complexity and extra cost to the lower unit assembly.
In addition, at least some known marine engine stern drives include dual propeller arrangements including an inner propeller shaft and an outer propeller shaft. A separate driven gear is provided to drive each of the inner and outer propeller shafts in opposite directions. Further, in at least some dual propeller arrangements, different speed propellers are utilized for increased performance and manueverability of watercraft. For example, one bevel drive pinion having two sets of teeth with different pitch diameters is utilized. Alternatively, two different pinions mounted to a single shaft are engaged with two different oppositely facing driven gears of different sizes. In this alternative configuration, one gear has a larger pitch diameter and more teeth than the other gear so that the larger gear rotates slower than the other gear. However, an increased size of one driven gear and a dual pitch pinion configuration result in a larger and more expensive gear case assembly.
In an exemplary embodiment, a drive train includes a bevel drive pinion gear having a number of teeth and a diametral pitch, a first driven gear having a number of teeth and a diametral pitch, and second driven gear having a number of teeth and a diametral pitch. As is conventional, the drive pinion gear and the first driven gear have equal diametral pitches. The diametral pitch of the second driven gear, however, is different than the diametral pitch of the pinion gear and the first driven gear. The first and second driven gears are simultaneously engaged with the drive pinion gear and rotate about a common axis, and because of the different diametral pitches of the first and second driven gears, a rotational output of the first driven gear is different than the rotational output of the second driven gear. Thus, for example, when applied to a single propeller marine propulsion system at a given engine speed, a propeller can be caused to rotate at a faster rate when operated in a reverse driven gear than when operated in a forward driven gear.
More specifically, the second driven gear has at least one or two less gear teeth than the first bevel driven gear which allows the second driven gear to rotate faster at a given speed than does the first driven gear, yet allows an adequate driving engagement despite the different diametral pitches of the first and second driven gears.
In exemplary embodiments for marine propulsion units, the drive train is configured for a single propeller outboard motor in a first embodiment and for a dual propeller stern drive in a second embodiment. The single propeller outboard motor embodiment includes a unitary shaft with the first and second driven gears selectively engageable with the propeller shaft. Therefore, a propeller is rotated faster in reverse for a given engine speed by changing a ratio of a reverse driven bevel gear and bevel drive pinion, but without adding a second bevel drive pinion or second set of bevel drive pinion teeth of different pitch diameters. By proper selection of the number of teeth on the reverse driven gear, reverse propeller thrust can be better optimized.
The dual propeller stern drive embodiment includes an inner propeller shaft and an outer propeller shaft, with a first driven gear attached to the inner propeller shaft and a second driven gear attached to the outer propeller shaft. A propeller is attached to each of the inner and outer propeller shafts. Both the driven gears are simultaneously driven by a single bevel pinion gear, and since the driven bevel gears have different numbers of teeth, the propellers rotate at different speeds. Because both driven bevel gears are the same diameter, however, gear case size is not affected. Cost and complexity of the bevel drive pinion are unaffected because a single bevel drive pinion with only one set of teeth is required. Increased motor performance is therefore provided at minimal cost, size, and complexity for a dual propeller stern drive.
In both single and dual propeller embodiments a unique gear development procedure permits two bevel gears of the same pitch diameter, but having different numbers of teeth, to engage a single bevel drive pinion with a single set of teeth having a single diametral pitch. A first gear machining summary, i.e., a listing of gear generator machine settings, is made of a drive pinion and a driven gear to serve as a baseline. A second gear machining summary is next created of another drive pinion and a driven gear in which the driven gear pitch diameter is set equal to the driven gear pitch diameter of the first machining summary but the driven gear has one or two less teeth. Gear generator machine settings for the desired reduced-tooth driven gear are approximated by averaging the first and second gear machine settings for driven gear, and particularly the settings that control tooth pressure angle are averaged. This averaging primarily modifies the pressure angle for the desired reduced-tooth driven gear which enables the driven gear to mesh effectively with the drive pinion of the first gear machining summary even though the diametral pitches of the drive pinion and reduced-tooth driven gears are different. Final gear machine settings are then selected by adjustment of the averaged driven gear settings to achieve an acceptable tooth contact pattern and driving arrangement between the drive pinion of the first gear machining summary and the reduced-tooth driven gear.